1. Field of the Invention
This invention relates to fuel assembly, channel box, production method of the channel box and core of nuclear reactor. More particularly, the present invention relates to fuel assembly, channel box, production method of the channel box and core of nuclear reactor which will be suitable for a boiling water reactor.
2. Description of the Prior Art
Each fuel assembly used in a boiling water reactor is equipped with upper and lower tie plates, a plurality of fuel rods whose both end portions are supported by these tie plates, fuel spacers for bundling these fuel rods and a channel box encompassing the bundle of fuel rods and fitted to the upper tie plate. The fuel assembly is loaded into the core of the nuclear reactor. A pressure loss is different inside and outside the fuel assembly during the reactor operation and a pressure difference occurs between the inner and outer surfaces of the channel box. This pressure difference is greater at the lower end portion of the fuel assembly and the pressure acting on the inner surface of the channel box is greater than the pressure acting on its outer surface. Therefore, the channel box expands outward and undergoes large creep deformation with the passage of the operation time of the nuclear reactor. This creep deformation will possibly result in the increase in a flow rate of leaking cooling water from between the channel box and the lower tie plate and in the trouble of a control rod operation.
One of the methods to cope with creep deformation of the channel box disposes recesses (or projections) on the sidewalls of the channel box as described in Japanese Patent Laid-Open Nos. 58487/1975 and 13894/1979 and in U.S. Pat. No. 3,715,274.
Japanese Patent Laid-Open No. 58487/1975 discloses a channel box equipped on its sidewalls with projections that project inward and thinly in a horizontal direction. Rigidity of this channel box is improved in a direction orthogonal to an axis. U.S. Pat. No. 3,715,274 teaches to dispose corrugated portions having concavo-convexities that repeat in an axial direction, on the sidewall portions of the channel box opposing the lower tie plate. Japanese Patent Laid-Open No. 13894/1979 shows in its FIGS. 3 and 4 a channel box equipped with projections which project inward and are disposed above, and in the proximity of, the upper surface of the lower tie plate and at a position of 1/3 of the full length of a fuel rod from the lower end of the fuel rod. The number of these projections is the same as that of a large number of tabs disposed on a band of a fuel spacer, and they are disposed at the same level and have a width capable of passing through the tabs. These tabs come into contact with the inner surface of the channel box and support the fuel spacer in a horizontal direction. FIGS. 7 and 8 of Japanese Patent Laid-Open No. 13894/1979 show the structure wherein a large number of projections described above are disposed on the sidewall portions of the channel box facing the lower tie plate. The lower tie plate is equipped on its outer side surface with a large number of grooves into which the projections described above are fitted.
In accordance with Japanese Patent Laid-Open No. 58487/1975 and U.S. Pat. No. 3,715,274, the height of the projections from the inner surface of the channel box in a direction vertical to an axis is limited to a range such that the projections can pass through the gap between the inner surface of the channel box and the support tabs disposed on the fuel spacer. When outwardly projecting projections are dispose on the channel box, too, the height of the projections cannot be much increased in order to avoid interference with a control rod.
As higher burnup of a fuel assembly and reutilization of a channerl box have been attempted recently, the residence time of the channel box inside a core tends to remarkably increase. Therefore, the increase in strength for inhibiting creep deformation of the channel box has been all the more desired than in the prior art. The channel box disclosed in Japanese Patent Laid-Open No. 58487/1975 and U.S. Pat. No. 3,715,274 cannot satisfy this requirement because the height of the projections cannot be increased much more than the height described above.
The projection disclosed in Japanese Patent Laid-Open No. 13894/1979 has a width such that the projection can pass through the gap between the tabs disposed on the fuel spacer. Accordingly, the height of this projection can be made greater than that of the two prior art references described above. However, the width of each projection positioned at the same level is limited by the gap between the tabs in the horizontal direction. Therefore, the degree of the increase in strength by the projection in Japanese Patent Laid-Open No. 13894/1979 is not much great and fails to satisfy the requirement described above. Furthermore, in accordance with the structure disclosed in Japanese Patent Laid-Open No. 13894/1979, each of the projections disposed on the channel box must be passed through the gap between the tabs disposed on each of a plurality of fuel spacers disposed in an axial direction when the channel box is fitted to a fuel bundle. Therefore, fitting of the channel box is troublesome and time-consuming.